A construction machine such as a hydraulic excavator and a wheel loader, a delivery vehicle such as a bus, cargo truck, dump truck and on-road truck, and a vehicle and a machine including a diesel engine (e.g., a stationary generator) and the like include a Selective Catalytic Reduction (hereinafter, referred to as “SCR”) system using a urea solution in a form of a device configured to control, within a regulatory value, nitrogen oxides (NOx) that are one of restricted substances contained in an exhaust gas discharged from an engine.
This urea SCR system includes: a tank configured to store a urea solution as a reduction-causing agent; a squeeze pump configured to pump the reduction-causing agent drawn from the tank; a reduction-causing agent supply passage through which the pumped reduction-causing agent is supplied; and a reduction-causing agent injection nozzle configured to supply the reduction-causing agent into an exhaust pipe from the reduction-causing agent supply passage.
An injector of the urea solution in the urea SCR system is exemplified by an air assist injector using a compressed air and an airless injector without using a compressed air. In order to raise a pressure of the urea solution, the airless injector includes a valve for opening and closing an aperture of a nozzle. With this arrangement, an injected amount of the urea solution can be controlled with a high accuracy by controlling a time and a frequency of opening and closing the valve.
In the urea SCR system, when a solvent (water) of the urea solution remaining inside the reduction-causing agent injection nozzle is evaporated after the engine is stopped, urea is crystallized. This causes a poor operation of an injection valve and a clogged injection hole, resulting in a faulty injection of the urea solution. Further, since a volume of the urea solution is increased when the urea solution freezes at a low temperature, a device such as the injection valve is likely to be broken.
In order to solve this problem, there has been known a reduction-causing agent supply device for preventing the urea solution from being solidified, the reduction-causing agent supply device including the squeeze pump configured to further supply the urea solution to the reduction-causing agent supply passage and the reduction-causing agent injection nozzle after the engine is stopped, and subsequently draw back and collect the urea solution, thereby collecting heat of the reduction-causing agent injection nozzle, so that the urea solution remaining in the injection nozzle is avoided from being heated (see, for instance, Patent Literature 1).